The invention relates generally to electrical interconnection of electrical components to substrates, in particular flip-chip reflow soldering and specifically the development of encapsulant-forming compositions.
Epoxy resin compositions have been used as semiconductor device encapsulants for over 25 years as noted by reference to U.S. Pat. No. 3,449,641, granted Jun. 10, 1969.
Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturing methods are described in U.S. Pat. No. 4,999,699, granted Mar. 12, 1991, and U.S. Pat. No. 5,250,848, granted Oct. 5, 1993. Encapsulant-forming compositions are applied after electrical interconnection.
The application of an encapsulant-forming (encapsulating), composition prior to interconnection by reflow soldering, wherein electrical interconnection occurs in the presence of an encapsulating composition is described in U.S. Pat. No. 5,128,746, granted Jul. 7, 1992. In U.S. Pat. No. 5,128,746, flip-chip production methods are described where electrical interconnection is achieved by adding a fluxing agent to a mixture of epoxy resin and curing agent prior to cure. During reflow soldering, the fluxing agent is activated and the resin is cured.
The use in the prior art of cross-linking agents having flux properties is found in PCT International Publication Number WO 93/06943, published Apr. 15, 1993 and its U.S. counterpart, U.S. Pat. No. 5,376,403, granted Dec. 27, 1994. In the publication, enhanced sintering is described using a protected cross-linking agent with fluxing properties in a metal powder and epoxy resin system where the solder is used to form the conductive film. Solder powder addition is used to sinter the metal powder, typically copper or silver, before setting the resin in order to create solid electrically conductive bridges between the powdered metal particles.
The use of synthetic thermosetting polymer resins together with soldering flux agents is described in U.S. Pat. No. 3,791,027, granted Feb. 12, 1974. Therein epoxide resin compositions are described wherein fluxing agents react with the epoxide resin to strengthen solder joints.
Electrically conductive adhesive compositions in which solder powder, a chemically protected cross-linking agent with fluxing properties and a reactive monomer or polymer (inclusive of epoxy resins) are described in U.S. Pat. No. 5,376,403, granted Dec. 27, 1994.
In the present invention, the fluxing additive of U.S. Pat. No. 5,128,746 is eliminated, while the function of fluxing is retained by selecting a cross-linking agent that has the property of also operating as a fluxing agent. In the flip-chip production method of the present invention, where electrical interconnection occurs within the encapsulating composition, the preferred embodiment of the invention resides in selecting a combination of dual functioning cross-linking agent and thermosetting resin or combination of such agent and thermosetting resin with a selected catalyst and controlling the sequence of flow soldering and gel formation to avoid inhibition of soldering. This is accomplished by providing an encapsulant composition, which, at surface mount reflow profile conditions, in which the gel formation (gel point is reached) after solder melt; that is, the gel is formed/gelation occurs after reflow soldering, whereby soldering is not inhibited.
As noted by reference to xe2x80x9cMANUFACTURING TECHNIQUES FOR SURFACE MOUNTED ASSEMBLIES,xe2x80x9d Wassink, R. K. and Verguld, M. F., 1995 ELECTROCHEMICAL PUBLICATIONS, LTD., soldering methods (and equipment) have converged from various IR soldering concepts to one main method, namely, hot-convection soldering. Besides this method other methods are used, but only in specific cases, such as resistance soldering for outer lead bonding of TAB and for soldering on foils.
In Wassink et al., at pages 275, 276, a typical profile for reflow soldering is described. A hot-air convection soldering oven having a number of zones whose temperature can be controlled separately is used in order to attain the desired temperature profile along the length of the entire oven. Such profile enables all joint areas to reach the soldering temperatures with limited temperature differences between the joint areas of components with different thermal mass.
Wassink et al. describes the typical three step heating approach of the prior art frequently used in reflow soldering using multiple hot-air convection ovens.
As also described in Wassink et al., the three steps are:
(i) starting with rapid heating to bring heat into the product (this reduces the length of the oven);
(ii) second step concerns temperature equalizing, i.e., to reduce the temperature differentials; usually a kind of temperature plateau for the hottest parts is pursued while the temperature rise of the coldest parts is chosen to be relatively slow; the effectiveness of this step can easily be assessed by the temperature differentials that exist on the assembly just before it enters the next step;
(iii) final rapid heating and subsequent cooling.
As further described in Wassink et al., each limit of the profile is determined by the maximum allowable thermal load of one of the parts of the assembly to be soldered.
The maximum (peak) temperature is determined by the base material of the printed board. Higher temperatures than 280xc2x0 C. will cause delamination. (Note: In most cases the printed board is the hottest component.)
The minimum soldering (peak) temperature is determined by the wetting of component metallizations.
The maximum time and temperature of the equalize region is determined by the solder paste. In the case of too heavy a thermal treatment, the activator (flux) in the solder paste will be consumed already at this stage of the process.
The time for which the solder is in the molten stage (in combination with a maximum temperature) is restricted by the formation of intermetallic layers inside the soldering joint. These layers make the soldered joint more brittle.
The specific values of the mentioned boundaries are determined based on the components and board material used.
A surface mount reflow profile for a 63 Sn/37 Pb solder illustrating the typical ranges is shown in FIG. 4.
In U.S. Pat. No. 3,791,027 (xe2x80x9cAngeloxe2x80x9d), the disclosure of which is incorporated herein by reference thereto, polymers and other materials which contain chemical functionalities; such as, amide, amino, carboxyl, imino, and mercaptan; which serve as flux agents are described. When soldering metals, these materials can be combined with materials which contain other functionalities; such as, epoxide and isocyanate to produce thermosetting polymers. Angelo describes three polymer categories in his invention which are set forth below.
1. Chemical functionalities; such as, carboxy terminated polybutadiene and carboxy terminated polyisobutylene, which, when used alone, do not harden and are easily removed with solvents. These are in essence fluxes and contain the same chemical functionalities found in traditional soldering fluxes.
2. Formulations that are non-crosslinking and can be softened or melted with the addition of heat. Examples cited in Angelo include Versarid 712 and Acryloidat 70. Since cross-linking does not take place, these formulations are similar to standard rosin or resin based fluxes frequently used in reflow soldering which contain chemical functionalities such as amino, carboxyl, amide, etc. Thus the same chemical functionalities are present both traditional rosin and resin fluxes and in the examples cited in Angelo which do not chemically cross-link to form thermoset polymer and hence may be removed by using a solvent or may be reheated and remelted to enable resoldering of solder joints.
3. Combinations of materials, which contain the chemical functionalities necessary to promote solder wetting; such as, carboxy, amino, etc., and materials that react chemically to form thermosetting polymers that cannot be easily removed through use of a solvent or reheated and remelted. Specifically Angelo shows examples of combinations of materials which contain such functionalities with epoxy resin materials which, when heated, form cross-linked networks which are not easily removable or cannot be remelted. Angelo cites the usefulness of such combinations to reside in their ability to reinforce the strength of the solder joint in situations when there is a low probability that a solder joint will need to be resoldered.
Pennisi, et al., U.S. Pat. No. 5,128,746, also describes the use of combinations of materials which contain chemical functionalities known to serve as fluxes and materials; such as, epoxy resins, which when reacted with the addition of heat, form chemically cross-linked polymers which add strength to solder joints and are not easily removable. Although Pennisi describes the function of the epoxy thermoset polymers as providing environmental protection to the flip-chip, the epoxy encapsulant described by Pennisi is also known to strengthen the fragile solder joints. Pennisi lists flux agents; such as, malic acid and other dicarboxylic acids that remove metal oxides and promote solder wetting. In essence, a material, malic acid, containing the carboxyl functional group, which is known to promote solder wetting, is combined with materials, epoxy resins, that form cross-linking, thermoset polymers.
In a third example, described in Capote, U.S. Pat. No. 5,376,403, a material containing a chemical functionality, such as carboxyl, known to assist in solder wetting, is combined with materials that form cross-linking thermoset polymers that are used in ink systems that assist in the fusing of low melting alloy powders with high melting metals and assist in the adhesion of the resultant metal network to a substrate.
In each case (Angelo, Pennisi, Capote) in which a material containing a chemical functionality, known to promote solder wetting, is combined with materials such as epoxy resins that form thermoset polymers, a method of heating is described in which the assembly is heated rapidly above the solder melt point. The application of temperatures above the solder melt point 183xc2x0 C. is critical as the solder must liquify in order to wet the surface metal.
As thermosetting polymers are initiated by the application of heat in order to stimulate cross-linking reactions, it becomes necessary to understand the cure kinetics involved in the curing of the material combinations selected. By chemically protecting the cross-linking material of the combination, Capote ensures that the cross-linking reactions are delayed and appropriate during the rapid heating process described in his invention.
Similar heating methods are described in Angelo and Pennisi, who both describe the application of heat during the soldering process as quick and rapid. As described previously in Wassink et al., a three step heating profile is typically used to solder electronic components to substrate boards. Rapid heating, as called for in Angelo, Pennisi and Capote, would adversely affect the parts and assemblies during soldering. This includes damage to components at high thermal excursion rates.
Thus, one frequently finds the heating step to be done using multizone ovens which allow materials in assemblies to achieve thermal equilibrium at temperatures above room temperature but lower than the solder melt point (183xc2x0 C.) in order to reduce thermal shock and subsequent damage. In SMT, this heating process in known as a surface mount reflow profile.
Therefore, in using combinations, as set forth by Angelo, Pennisi and Capote, in which materials containing chemical functionalities; such as carboxyl and amino, known to promote solder wetting, are combined with materials that form cross-linked thermoset polymers through the addition of the heat, heating processes are used that do not involve a rapid heating rate to the solder temperature but instead allow materials to be used in the final assembly to reach a thermal equilibrium. Above room temperature but lower than soldering temperature, it becomes critical to understand the cure kinetics of the combination of thermosetting materials, in view of the desired non-rapid heating profile in order to prevent significant crosslinking of the combination prior to solder melt point.
The present invention relates to thermosetting resin encapsulating formulations in which the cross-linking agent functions as flux for reflow soldering. Included in such formulation are systems in which the cross-linking of resin with such dual purpose cross-linking agent is effected by a catalyst. In the present invention, the cross-linking agent acts as a fluxing agent at reflow soldering conditions and acts as a cross-linking agent in the involved resin system in a manner such that soldering is not inhibited by premature gelling.
In systems of the present invention, where electrical connections along a single axis are involved and reflow soldering is effected in a liquid resin system, the sequencing of stages of cross-linking vis-a-vis reflow soldering is critical.
It is essential that the gel point of the system not be reached prior to formation of the interconnection by liquid solder which interconnection occurs at reflow solder temperature.
It has been observed that where the gel point is reached before melting of the solder, the solder does not wet and does not effectively engage the opposing locus where the electrical interconnection is to be made because solder flow is restricted.
It is believed that the criticality of this requirement in the system of the present invention derives from the inability of the solder to flow in a resin system where the gel point has been reached. It is believed that where the resin system enveloping the solder is liquid, even where the viscosity is great, the melted solder traverses and bonds to the opposing target area and is fluxed by the cross-linking agent. In fact, where a mixed phase exists, wetting, bonding and fluxing occur; however, once the gel point is reached and melt occurs, observation of the cured product demonstrates the failure of successful interconnection and the failure of wetting and flow.
One aspect of the present invention relates to an epoxy resin based encapsulant-forming composition for use in reflow soldering of an electrical component to a substrate. The composition forms an acid anhydride epoxy resin system. The fluxing encapsulant composition is composed of an epoxy resin, an anhydride cross-linking agent for the resin that also functions as a fluxing agent at reflow soldering conditions and a catalyst. The combination of compounds comprising the encapsulant composition is selected to provide a composition which does not reach gel point prior to formation of electrical interconnection at said reflow soldering conditions.
The thermosetting resin is preferably an epoxy resin and the cross-linking agent is selected from anhydrides that intrinsically possess the added activity of fluxing at reflow soldering temperatures.
Referring specifically to eutectic tin/lead soldering temperatures (183xc2x0 C.), examples of anhydrides that serve as fluxes include succinic anhydride, methyltetrahydrophthalic anhydride, polyadipic polyanhydrides, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, polyazelaic polyanhydrides, and admixtures thereof. The fluxing agent of choice is methyltetrahydrophthalic anhydride and the catalyst-containing encapsulant composition of choice is tin octoate.
Another aspect of the invention relates to a method of attachment and encapsulation of integrated circuits, such as flip-chips or ball-grid arrays, wherein reflow soldering is effected by the encapsulant-forming composition. It is essential that the catalyst promotes gel formation during or after soldering, but does not form a resin gel (a gel at or past gel point) before soldering in order not to inhibit soldering by formation of the gel prior to soldering.
Yet another aspect of the present invention relates to an electrical component having a plurality of electrical terminations, each termination is composed of a solder bump; a component carrying substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component; and, an encapsulating material that removes metal oxides from the surfaces of the electrical terminations of both the component and substrate. Such encapsulating material is preferably composed of an epoxy resin, an anhydride that functions as a fluxing agent to remove the oxide coatings from the component terminations and the substrate terminations prior to and during soldering and also reacts with the epoxy. The gel point of the encapsulant-forming composition is reached at or above the soldering temperature. In the assembly, the encapsulating material is disposed between and fills the opening between electrical component and substrate. This may be done either by dispensing encapsulant on the board and then pressing the component onto the encapsulant or by dipping the component in the encapsulant prior to placement on the board. After heating to soldering temperature, the solder is reflowed and electrically connects the electrical component to the substrate. The catalyst present in the encapsulation composition promotes gel formation during or after soldering so that soldering is not inhibited by premature gelling. In other words, fluxing and reflow soldering occur prior to resin system gel point.
The physical properties of the cured epoxy encapsulant include such measurable properties as glass transition temperature, tensile strength, modulus, dielectric strength, and dissipation factor. These properties affect reliability of the final encapsulated component. The application of the product containing the encapsulated component determines the physical properties that the encapsulant must possess.
For example, flip-chip components typically require glass transition temperatures at or above 120xc2x0 C. The choice of the resin and anhydride components will therefore be restricted to compositions which yield glass transition temperatures at or above 120xc2x0 C. DGEBA resins, EEW 180-190 are among the resins of choice for this application.
The choice of the resin in the system depends on the desired properties of the final product. The functionality of the resin and its chemical structure will influence the cross-link density of the cured system. Typically, DGEBA or novolak resins are preferred with anhydride cured systems. Epoxy resins based on methylene dianiline; such as, MY720 from Ciba Geigy or resins containing an amine will cause premature gelation before soldering, since the amine acts as a catalyst. Therefore, these latter epoxy resins are unacceptable for use in the present invention.
The present invention involves the use of compositions which function as flux and hardener for thermosetting resins and in particular epoxide resins, the resins of choice, in encapsulating formulations which find application in and are adopted for use at surface mount reflow soldering profile conditions adapted for assembly architecture and composition of the bond sites. The encapsulants of the present invention are formulated based on reflow temperature profile and the bond site composition which, after melt, forms the electrical interconnection.
Thus in accordance with the present invention the soldering profile in such instances includes heating the assembly components to a temperature below the soldering temperature to prevent damage to the assembly parts that are susceptible to injury at high thermal excursion rates. This heating step is dependent on the size, mass and materials used in the assemblies and is referred to in the art as the soak or equilibrium stage of the profile. In connection with computer mother boards, telecommunications equipment and panels of smaller assemblies, the target temperature at the end of the soak step preferably approaches the solder melt point and may range from about 20xc2x0 C. below the solder melt point up to the melt point thereof. The period of application of heat to achieve the target equilibrium temperature typically varies and may be from about 30 to about 120 seconds for large surface mount assemblies. The soaking step is preceded by a rapid increase in temperature, called the ramp step; the rate of increase in temperatures is selected to achieve maximum heat input without injury such as microcracks, warping and the like and is dependent on the involved architecture and materials. In the case of typical large surface mount assembly temperature increases of about 1xc2x0 C. to about 4xc2x0 C. per second prior to soak are the norm. The final step involves a rapid heating step following soak where the assembly is quickly heated to a temperature above the solder powder melting point in order to form the metal interconnection between the assembly components. Typically one component of the assembly is an electrical component such as a packaged or unpackaged integrated circuit and the other component is a substrate for microelectronic circuitry on which the metal bond sites are located. In the art this step is referred to as the spike stage with a total time above melt point typically ranging from about 30 to 90 seconds. The peak temperature at any locus on the assembly is preferably in the 205xc2x0 C. to 235xc2x0 C. range.
For low thermal mass assemblies; that is, assemblies less than 0.062 inches thick and/or assemblies having an area typically 10 square inches or less, heat input is typically achieved using a smooth non-stepped ramp at the rate of 1xc2x0-4xc2x0 C. per second. Solder melt typically is achieved for low thermal mass assemblies in a time frame varying from something less than a minute up to about three minutes.
As the thermal mass of the assembly increases, slower heating rates and stepped processes are employed to bring such assemblies up to thermal equilibrium prior to reflow.
A criteria for selection and combination of thermosetting resin and cross-linking agent (whether in the presence of catalyst or otherwise) in order to achieve the requisite sequence of achieving solder melt before the gel point is reached resides in selecting a thermosetting resin and cross-linking agent combination that when heated using differential scanning calorimetry (xe2x80x9cDSCxe2x80x9d) displays initiation of exothermic reaction with the range of about 140xc2x0 C. to 180xc2x0 C. for lead tin eutectic mixture. Exothermic activity as observed using DSC correlates to cross-linking activity. The peak of said exotherm in turn correlates with the level of cross-linking that indicates gel formation.
Although in the final analysis gel formation for the specific combination is determined empirically and exceptions occur, the criteria for selection that is generally applicable is to choose polymer systems wherein exothermic activity, as observed using DSC, is preferably initiated at a temperature no lower than 40xc2x0 C. below the solder melting point and where the peak exothermic activity registered occurs at a temperature above the solder melting point.
Anhydrides, including, in particular, highly active polyanhydrides, are the preferred dual-functioning cross-linking and flux agents for the encapsulant of the present invention.
In addition to the aforesaid anhydrides, other cross-linking agents that are suitable for use in the present invention, though not necessarily with the same order of success, include materials containing chemical functionalities, such as carboxyl, amino, imino, amide, and mercaptan, as described in Angelo, that inherently act as fluxing agents as well as cross-linking agents.
Amine cross-linking agents that provide the requisite latency (gel point formation at or above the melting point of the solder powder), such as Aificurexe2x80x94LX-1 (manufactured by Ajinomoto Co., Inc., Tokyo, Japan), which is heterocyclic diamine having an active hydrogen equivalent 67, may be used.
Amides and other known nitrogen containing curing agents such as melamine, dicyandiamide, imidazoles, hydrazides, thioureas and the like may also be used as cross-linking agents.
Also useful as crosslinking agents are the well-known polydric phenols which are polycondensates of one or more phenols such as phenol, various alkylphenols and naphthols with aldehydes such as formaldehyde, acetaldehyde, acrolein, glyoxal, benzaldehyde, naphthaldehyde and hydroxybenzaldehyde or ketones such as cyclohexanone and acetophenone, vinyl polymerization type polyhydric phenols such as polyvinylphenol and polyisopropenylphenols, the polyhydric phenols of the present first invention, Friedel-Crafts type reaction products of phenols with diols such as those represented by the formula (1): 
Dialkoxy compounds represented by the following formula (2): 
or, dihalogens represented by the following formula (3): 
and Friedel-Crafts type reaction products of phenols with diolefins such as dicyclopentadiene and diisopropenylbenzene.
In assemblies where the inherent fluxing ability of the cross-linking agent is insufficient to form proper metallurgical solder connections, it may be necessary to add additional fluxing agents in order to increase the flux activity to the formulation.
To supplement the fluxing activity possessed by the encapsulant attributable to the combination of cross-linking agent and thermosetting resin, other known fluxes may be added in minor amounts so as not to adversely effect the properties of the cured thermoset polymer. Moreover, in choosing the fluxes, whether the basic cross-linking agents of the present invention with flux activity or the supplemental fluxing agents, low molecular weight compositions which generate outgassing at surface mount reflow soldering conditions utilized cannot be used since the integrity of the encapsulation is compromised when outgassing occurs by the presence of voids in the encapsulant. Accordingly, dicarboxylic acid flux agents which produce outgassing should be avoided.
Moreover, in selecting the supplementary fluxing agent, its effect on material properties, such as adhesion, tensile strength, moisture absorption or glass transition temperature must be considered. Accordingly, compounds appearing to be compounds of choice because of high flux activity, such as dicarboxylic acids, which reduce glass transition temperature and bulk physical properties of the final polymer network, need to be used, if at all, in amounts that do not affect the resultant polymer.
It has been found that the carboxy terminated polyesters used in the ink formulations of U.S. Pat. No. 5,439,164 are suitable as supplementary flux additives in the encapsulants of the present invention.
It has also been found, where the cross-linking agent, in the amounts used, has inadequate activity as flux to achieve the required level of oxide removal, that among the preferred additives are those that cross-link or bind chemically into the polymer matrix and do not retard the physical properties of the resultant polymer network. For this purpose carboxy terminated polyesters have been found to be a suitable flux additive.
The present invention relates to an encapsulant suitable for use at surface mount reflow profile conditions and a method of electrically connecting an electrical component having a plurality of metal bond site terminations to a component carrying substrate having a plurality of metal attachments sites corresponding to the terminations of said electrical component, at surface mount reflow profile conditions. These conditions vary based on the solder used on the solder bumps, typically forming the metal bond site on the electrical component and the composition and architecture of electrical component and substrate involved. The protocol followed is equivalent to that described in FIG. 4 herein. The encapsulant utilized is comprised of a thermosetting resin and a cross-linking agent that acts, below the solder melt temperature, as a fluxing agent which removes oxide coatings from the surface of the metal bond sites on said electrical component and on said component carrying substrate when heated to said solder melt temperature. The thermosetting resin and cross-linking agent combination has a gel point at or above the solder melt temperature, and preferably above the solder melt temperature. The method comprises the steps of: a) removing oxide coating from the surface of metal bond sites while the encapsulant is in liquid form; b) melting the solder at the metal bond sites to electrically connect the electrical component which has the plurality of metal bond site terminations to the component carrying substrate which has the plurality of metal bond site terminations corresponding to the terminations of said electrical component while said encapsulant is in liquid form and prior to said encapsulant reaching its gel point; then c) forming an encapsulant gel; that is, reaching the gel point. Thereafter the gel may be cured to form the encapsulant, thereby providing protection and enhancing the mechanical connection.
The thermosetting resin is preferably an epoxide. The preferred cross-linking agents are a polyanhydride and carboxy terminated polyesters.
The present invention also relates to a method of producing a low thermal mass assembly by, simultaneously, during surface mount reflow soldering, encapsulating and electrically connecting an electrical component having a plurality of metal bond site terminations to a component carrying substrate having a plurality of metal attachments sites corresponding to the terminations of said electrical component utilizing an encapsulant comprised of a thermosetting resin and a cross-linking agent that acts, below the solder melt temperature, as a fluxing agent which removes oxide coatings from the surface of the metal bond sites on said electrical component and on said component carrying substrate. The thermosetting resin and cross-linking agent combination has a gel point at or above, and preferably above, the solder melt temperature. In this method the low thermal mass assembly is ramped at the rate of about 1xc2x0 C. to about 4xc2x0 C. per second for a time sufficient to achieve solder melt and electrical interconnection. The steps of the method occur sequentially during ramping, as follows:
1. oxide coating is removed from the surface of the metal bond sites, while the encapsulant is in liquid form;
2. The solder, which may comprise one or the other bond sites is melted to electrically connect the electrical component having a plurality of metal bond site terminations to the component carrying substrate having a plurality of metal attachment sites corresponding to the terminations of said electrical component (the encapsulant is in liquid form during this step and solder melt occurs prior to the encapsulant reaching its gel point); then
3. The encapsulant gel is formed at the gel point. Thereafter, the resultant gel is cured to encapsulate said electrical solder connections. The preferred thermosetting resin is an epoxide. The preferred cross-linking agents are polyanhydride and carboxy terminated polyesters.
Yet another aspect of the present invention relates to a specific encapsulant for encapsulating and electrically connecting a metal bond site of a first electrical component to a metal bond site of a second electrical component at surface mount reflow conditions. The encapsulant comprises an epoxy resin; a cross-linking agent for the resin that also acts as a fluxing agent that removes oxide coatings from the surface of the first and second electrical components; and a catalyst for catalyzing cross-linking of said epoxy resin with said cross-linking agent, the peak exotherm of said mixture of catalyst, epoxy resin, and cross-linking agent as measured using DSC at a ramp rate of 10xc2x0 C. per minute being at or above the solder melting point whereby the gel point of said cross-linked epoxy resin is reached after solder melt. The preferred catalyst is tin octoate. The preferred cross-linking agents are polyanhydrides.
Another aspect of the present invention relates to a method for selecting the components of the encapsulant wherein the encapsulant is comprised of a thermosetting resin, a cross-linking agent that also acts as a flux and optionally a cross-linking catalyst which comprises: a) identifying the melting point of the involved solder at the metal bond sites; b) heating the encapsulant composition to a temperature exceeding the melting point of such solder; c) measuring the exothermic activity of the encapsulant during heating step b); d) identifying the temperature at which peak endothermic activity occurs; and e) selecting as an encapsulant an encapsulant that displays peak exothermic activity at or above the melting point of the solder.
The selection method suitably also includes identification of the initiation of exothermic activity and selection of the encapsulant also based on initiation of thermal reaction. The encapsulant selected is one that wherein initiation of exothermic activity occurs at a temperature that is no lower than 40xc2x0 C. and preferably no lower than 30xc2x0 C. below the solder melt temperature.